The present invention relates to a method for preparing porous organic-inorganic hybrid materials and their catalytic use. More particularly, the present invention relates to porous organic-inorganic hybrid materials containing metal and a method for preparing the materials. That is, it relates to a novel method for preparing porous organic-inorganic hybrid materials containing metal such as iron, which can be used not only for adsorbents, gas storing materials, sensors, catalysts and catalyst carriers, etc., but also for including guest molecules smaller than the pore size or separating molecules larger than pore size, since these porous organic-inorganic hybrid materials have a large surface area and a pore of a molecular size or nano size, wherein the organic compound ligand is bonded to a central metal by the preparing porous organic-inorganic hybrid materials containing metal such as iron by irradiating microwaves instead of heat treatments such as the conventional electric heating, etc. as a heat source of the hydrothermal or solvothermal synthesis reaction after forming crystal nuclei by pre-treating a metal or metal salt and an organic compound in the presence of a solvent. Also, the method for preparing porous organic-inorganic hybrid materials of the present invention further comprises a step of purifying the obtained porous organic-inorganic hybrid materials by treating them with an inorganic salt. Further, the present invention relates to a novel method for preparing porous organic-inorganic hybrid materials wherein use of hydrofluoric acid has been eliminated and the porous organic-inorganic hybrid materials obtained by said preparation method can be used as a heterogeneous catalyst.
The porous organic-inorganic hybrid materials prepared according to the present invention can be defined as porous organic-inorganic polymer compounds formed by binding a central metal ion with an organic ligand, wherein the central metal ion may be an iron ion. The compounds are a crystalline compounds having a pore structure of a molecular size or nano-size and containing both an organic and an inorganic compounds within the framework structure.
The term “porous organic-inorganic hybrid materials” has a broad meaning, and in general, it is also referred to as “porous coordination polymers” [Angew. Chem. Intl. Ed., 43, 2334 (2004)], or “metal-organic frameworks” [Chem. Soc. Rev., 32, 276 (2003)].
At present scientific research is focused on materials developed by integrating molecular coordination bonding with material science. Said materials have large surface area and pores of a molecular size or nano size, and thus can be used not only for adsorbents, gas storing materials, sensors, membranes, functional thin films, catalysts and catalyst carriers, etc., but also for including guest molecules smaller than their pore size or separating molecules depending on sizes of the molecules by using their pores. Thus, they have gained much importance.
Porous organic-inorganic hybrid materials have been prepared by various methods. Usually, they have been prepared by a hydrothermal synthesis reacting at high temperature by using water as a solvent or by reacting near room temperature using solvent diffusion, or a solvothermal synthesis using an organic solvent [Microporous Mesoporous Mater., 73, 15 (2004); Accounts of Chemical Research, 38, 217 (2005)].
Such porous organic-inorganic hybrid materials have been prepared through crystallization at a synthesis temperature of at least the boiling point of a solvent or solution mixture under an autogeneous pressure using water or a suitable organic solvent, as in methods for preparing other inorganic porous materials such as zeolites or mesoporous compounds.
The hydrothermal preparation methods mentioned above have problems such that an excessive amount of energy is consumed because usually a reaction time of at least several days is required for obtaining complete crystalline organic-inorganic hybrid materials since the nuclei formation or crystallization process is very slow. In particular, it shows a very low efficiency since the reaction is proceeded only by a batch-type manner [Angew. Chem. Intl. Ed. vol. 42, p. 5314 (2003); Angew. Chem. Intl. Ed. vol. 43, p. 6296 (2004)].
In order to overcome these problems, a method to maximize the efficiency by applying microwaves to the synthesis of porous inorganic materials including zeolites has been suggested, and methods applying microwaves were known in some patents and articles published since 1988 [U.S. Pat. No. 4,778,666; Catal. Survey Asia, vol. 8, p. 91 (2004)]. In many cases, it has been reported that the synthesis time has been decreased and porous inorganic materials can be continuously synthesized when microwaves were used compared to the hydrothermal synthesis by conventional heating.
Unlike the porous inorganic materials above, porous organic-inorganic hybrid materials have been recognized to have unique features that they can be used not only for catalysts, catalyst carriers, adsorbents, ion exchanging materials and gas storing materials, but also for storing, preparing and separating nanomaterials, and for nanoreactors, due to their characteristics such as large surface area, crystalline structure of a very high regularity and relatively high thermal stability, etc. In this regard, Cr-MIL-100, which is an organic-inorganic hybrid material of MIL-100 structure (MIL: Materials of Institute Lavoisier), has been reported [Bulletin of Korean Chemical Society vol. 26, p. 880 (2005)].
However, as for the organic-inorganic hybrid materials containing Cr as stated above, due to the Cr component that is harmful to the human body, its use is relatively limited. In particular, iron-organic-inorganic hybrid materials having Fe as a central metal which is not harmful to the human body cannot be easily formed by the synthetic method of the organic-inorganic hybrid materials containing the Cr component, and thus development of a novel preparation method for said materials has been needed.
Also, as for the synthesis of porous organic-inorganic hybrid materials by hydrothermal synthesis, in general, a mixed-acid comprising nitric acid, hydrofluoric acid, etc. is used in order to regulate the rate of forming crystals. As for representative porous organic-inorganic hybrid materials prepared by the hydrothermal synthesis, MIL-100 (Cr) represented by formula of Cr3O(H2O)2F[C6H3—(CO2)3]2.nH2O (n˜14.5) and MIL-101 (Cr) represented by formula of Cr3F(H2O)2O[C6H4(CO2)2]3.nH2O (n˜25) have been reported [Science 23, 2040 (2005); Accounts of Chemical Research, 38, 217 (2005)]. The organic-inorganic hybrid materials of a metal-organic framework structure where the Cr component is substituted by another metal have not been reported yet. In the meantime, the organic ligand present in the porous organic-inorganic hybrid materials prepared by above conventional processes, brought about small surface area and pore volume.
Therefore, development of porous organic-inorganic hybrid materials that are environmental friendly, have large surface area, and are also expected to be practically used for industrial application the new future and a novel preparation method thereof has been keenly required.